Method of optimizing a response of a gas correlation radiometer to a trace amount of a target gas

ABSTRACT

A method optimizes gas correlation radiometer response to trace amounts of target gas in the free atmosphere in competition with interfering gas. Operations identify spectral regions of a first absorption spectrum of the target gas, and of a second absorption spectrum of the interfering gas. A set of similarity data is determined as a function of overlap regions within the spectral region, and a set of contrast data is determined as a function of non-overlap regions within the spectral region, including a plurality of data items within each of a plurality of bandwidths, and a data item corresponding to a center wavelength within each bandwidth. Graphs correspond to each bandwidth. From one graph a center wavelength of an infrared filter is selected, and from another graph there is selected a bandwidth of the infrared filter, to configure an infrared filter for use with the gas correlation radiometer.

RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 10/155,770, filed on May 25, 2002 now U.S. Pat. No. 6,750,453,entitled “METHODS OF AND APPARATUS FOR DETECTING LOW CONCENTRATIONS OFTARGET GASES IN THE FREE ATMOSPHERE”, by Dr. Loren D. Nelson and MartinJ. O'Brien (the “Parent Application”), priority under 35 U.S.C. 120 ishereby claimed based on the Parent Application, and such ParentApplication is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to detecting gases, and moreparticularly to methods of and apparatus for detecting trace amounts oftarget gases, such as natural gas, remotely along long-range paths inthe free atmosphere, wherein the exemplary natural gas detection isenabled by simultaneously remotely detecting methane and ethane alongthe same long-range path in the free atmosphere.

2. Description of the Related Art

In the field of gas detection, attempts are made to detect one specific,or “target”, gas even though local conditions render such detectiondifficult. Such difficulty may be based, for example, on the fact thatthe target gas may only be present in “trace amounts”, such as one or afew parts per billion (PPB). Moreover, the target gas may be mixed with,or present with, water vapor and/or undesired (non-target) background,or “competitive”, gases that may be in the same atmosphere as containsthe target gas, for example. The background gases are referred to as“competitive” gases because there are overlaps in absorption spectra ofthe trace gases and the background gases, e.g., in the infraredabsorption spectra of such gases.

Many industries require facilities to detect target gases, such thatthere is a general need for accurate, fast and cost-competitivedetection of target gases. However, the natural gas pipelinedistribution system is the largest chemical distribution system in theUnited States. As a result, although equipment for detecting targetgases other than natural gas has wide application in the United States,for example, the natural gas pipeline industry has the greatest need foraccurate, fast, and cost-competitive chemical leak detection equipmentand methods. This need relates in part to regulations that require gasutilities to perform periodic surveys for natural gas leaks.

Initially, in target gas detection for the natural gas industry, thereis a need to distinguish between natural gas as a target gas, and othercombustible gases. The main constituents of natural gas are methane andethane, with methane being the primary component. However, methane isproduced by many natural biological sources, including animal and plant.Thus, if an elevated methane level is sensed, it does not necessarilymean that there is a natural gas leak. In contrast, there are nosubstantial natural ethane emission sources. However, ethane generallydoes not exceed twenty-percent of natural gas. As a result, ethane isboth more difficult to detect, but is a better indicator of natural gasthan methane. Thus, to have an optimal natural gas detector, there is aneed for the detector to simultaneously detect both methane and ethaneto assure that the detected gas is from a natural gas leak and not froma natural emission source.

This need to simultaneously and independently detect both ethane andmethane is not met by current gas detection equipment. For example,flame ionization detectors (FID) cannot distinguish natural gas fromsuch competitive gases. As a result, when currently available FIDequipment is used in an attempt to detect natural gas, the FID equipmentprovides “false natural gas alarms” based on the detection of leakingpropane tanks, leaking gasoline cans, so-called “sewer gas”, and allother combustible gases. A natural gas pipeline utility using the FIDequipment must respond to each false natural gas alarm although there isin fact no natural gas leak. Another limitation of the FID equipment isthat during the detection process, it is generally necessary to placethe equipment very close to the ground and within a “cloud” of thetarget gas that is to be detected. As a result, the FID detectionprocess is relatively slow, and FID equipment cannot be used at a placeremote from the locale of the gas leak, for example.

Also, Fourier Transform Infrared (FT-IR) spectro-radiometers use aninterferometer to determine the spectral content of light passingthrough the free atmosphere. However, the output of such FT-IRinstruments is based on a combination of all of the gases that areoptically “active” (e.g., infrared absorptive) within the spectralregion of the instrument. Thus, their temporal response is generallypoor. Moreover, FT-IR systems are expensive, have very limited detectionrange through the free atmosphere, and cannot detect very lowconcentrations of target gases.

Further, in contrast to the FID and FT-IR techniques, tunable diodelaser absorption Spectroscopy (TDLAS), laser absorption spectroscopy(LAS), and differential absorption laser-based radar (DIAL) all uselaser emission sources that are narrow band. For example, the DIALdevices typically monitor only one or two very narrow spectralabsorption lines. Laser-based techniques are more costly to manufacture,maintain and use compared to broadband techniques such as gascorrelation radiometry (GCR). However, gas correlation radiometry (GCR)is generally a passive technique that relies on solar illumination orscattering, or on thermal emission background. Thus, GCR instruments donot have an active source of energy that is directed through the freeatmosphere to the instrument. Further, while GCR instruments may beprovided with filters that improve a signal to noise ratio by generallylimiting the overall bandwidth of light admitted to a detector of theGCR instrument, such filters do not provide an optimized bandwidtharound an optimized central bandpass wavelength. As a result, thesensitivity of such GCR instruments may be as low as 10 to 100 parts permillion (PPM).

Moreover, the FT-IR, TDLAS, LAS, DIAL and GCR technologies provideseparate background gas and target gas channels that are interrogatedsequentially. That is, light transmitted along a path through the freeatmosphere and then through the background channel may be detected by adetector first. After such detection, the light transmitted through thesame path through the free atmosphere and then through the target gaschannel is detected by the same detector. The resulting temporal, orsequential, spacing of the alternating detection of the backgroundchannel and the target gas channel may vary from 0.1 second to severalminutes. That is, it generally takes more than 0.1 seconds for thesesystems to provide a complete data set consisting of a target gasabsorption measurement and an atmospheric background measurement, andduring that time period, there may be changes in the atmosphericconditions along the path of the light. Thus, the light that istransmitted along the path and through the target gas channel may havebeen subjected to different atmospheric conditions along the light path(e.g., atmospheric turbulence and variability) than the lighttransmitted through the background channel. As a result, the accuracy ofthese instruments is subject to a sensitivity limitation when used in adynamic atmosphere. Atmospheric turbulence and variability generallylimit the ultimate sensitivity of these instruments in that the samevalue of instrument output provided at different times may not be basedon the same amount of the target gas. Further, attempts to avoid suchatmospheric-induced inaccuracies, e.g., attempts to distinguish betweensignals generated based on a target gas and on the varying atmosphericconditions, have generally been limited to situations in which light istransmitted only a few feet through a detection path that may containthe target gas to be detected. For example, it may be practical toprovide known modulation imposed on light transmitted along a detectionpath that is only a few feet long from transmitter to detector. Giventhe few feet between the transmitter and the detector, a conductor mayeasily input the characteristics of the know modulation to the detectorso that demodulation will be accurate. However, problems are faced inaccurately demodulating the modulates light when the detection pathmust, for practical purposes, be hundreds or thousands of feet long

In addition to the accuracy limitation due to limited sensitivity ofthese prior instruments that interrogate the separate background gas andtrace gas channels sequentially, such instruments have limitations whenattempts are made to use the instrument on a mobile platform, such as atruck or airborne vehicle. For example, by the very motion of the mobileplatform, there is a dynamic, or different, atmosphere through which thelight is transmitted. As a result, the problems of atmosphericturbulence and variability limit the ultimate sensitivity of theinstrument used on this type of platform. Remote sensing systems mountedon moving platforms have the additional problem of variable surfacereflectivity. Prior remote sensing instruments mounted on mobileplatforms suffered additional sensitivity limitations due to thetemporally sequential manner of monitoring separate background and tracegas channels.

Some have attempted to avoid these and other limitations of atmosphericturbulence and variability by isolating samples of the target gas in aclosed chamber that is controlled to avoid turbulence and variability.However, the need to capture such samples from a possible location of agas leak, and other time factors, render the local sampling of thetarget gas impractical for detecting trace amounts of target gases suchas natural gas along miles and miles of pipeline, for example.

Another problem faced in detecting target gases is isolation of a signalrepresenting the target gas from other signals caused by backgroundemissions. For example, although some types of gas detectors modulatethe light just before the light impinges on a light receiver to removethermal emission due to a warm instrument housing, such modulation atthe receiver does not remove other background emissions such asatmospheric turbulence, jitter, beam wander, changes in the index ofrefraction, or the constant thermal emission of the atmosphere and theEarth.

What is needed, then, in the general field of detecting gases, is a wayto distinguish between the presence of one target gas and other gasesthat are normally present in the free atmosphere at the same time and inthe same place as the target gas. What is also needed in target gasdetection, such as for the natural gas industry, is to be able todistinguish between natural gas as a target gas and other combustiblegases. Moreover, there is a need for an optimal natural gas detectorthat simultaneously detects both methane and ethane to assure that thedetected gas is from a natural gas leak, so as to avoid false naturalgas alarms based on the detection, for example, of the noted leakingpropane tanks, etc. In addition, there is a need to increase detectiondistance, that is, to increase the distance from a target gas detectioninstrument to a remote location of the target gas that is to bedetected, and to also increase the detecting speed of such instruments.As well, there is a need to provide an instrument that will have a highsensitivity independently of atmospheric turbulence and variability.Finally, there is a need to remove undesired influences from the targetgas signal so as to isolate detector signals representing the targetgas. Such undesired influences include, for example, stray atmosphericfluctuations, such as humidity, atmospheric turbulence, changes in theindex of refraction, and beam path variation; stray back light, such asthe constant thermal emission of the atmosphere and the Earth; andsystem influences, such as jitter, beam wander and drift, and variationof source illumination.

SUMMARY OF THE INVENTION

Broadly speaking, the present invention fills these needs by providing,for the general field of detecting gases, methods of and apparatus fordistinguishing between a target gas and other gases that are normally inthe free atmosphere at the same time and in the same place as the targetgas. The present invention fills the needs for trace gas detection inthe natural gas industry by an ability to distinguish between naturalgas as a trace gas and other combustible gases. The present inventionalso provides a more optimal natural gas detector that simultaneouslydetects both methane and ethane to assure that the detected methane isfrom a natural gas leak, so as to avoid false natural gas alarms basedon the detection, for example, of leaking propane tanks, etc. Thepresent invention also fills these needs by substantially increasing thedetection distance. That is, by the present invention the distance fromthe detection instrument to a location of the target gas that is to bedetected may be up to about fifty-three hundred feet. As a result,mobile platforms, such as trucks and aircraft, may be used to carry theequipment of the present invention during high-speed remote monitoringalong long detection paths. In addition, the equipment of the presentinvention is provided with high sensitivity independently of atmosphericturbulence and variability, and the above-described undesired influencesare removed from the trace gas signal so as to isolate detector signalsrepresenting the trace gas.

An embodiment of the present invention includes a method of optimizing aresponse of a gas correlation radiometer to a trace amount of a targetgas present in the free atmosphere along a detection path to the gascorrelation radiometer. The detection path may also contain at least onecompetitive other gas the presence of which in the free atmosphere mayinterfere with detection of the trace amount of the target gas. The gascorrelation radiometer uses an infrared filter for the response, and themethod includes an operation of determining an absorption spectrum ofthe target gas modeled according to field parameters. Another operationdetermines an absorption spectrum of the at least one competitive gasmodeled according to the field parameters. A further operationdetermines similarity and contrast between the absorption spectra of theatmosphere and of the target gas. Another operation determinesdifferences between respective values of contrast and similaritycorresponding to a plurality of band passes and center wavelengths ofpossible infra red, filters. For each of the plurality of band passes,another operation plots the differences as a function of centerwavelength. The infrared filter is optimized for use in the gascorrelation radiometer by an operation of selecting a combination ofinfrared filter center wavelength and band pass that results in thelargest value of contrast minus similarity for the trace gas present inthe free atmosphere along the detection path containing the at least onecompetitive other gas. A related aspect of the method involves anoperation of mounting the optimized infrared filter in the gascorrelation radiometer.

Another aspect of the method is that the at least one competitive gas iswater vapor and the determining of the second absorption spectrumdetermines the second absorption spectrum corresponding to the watervapor.

A further aspect of the method relates to the at least one competitivegas being water vapor and another gas, wherein the determining of thesecond absorption spectrum determines the second absorption spectrumcorresponding to both the water vapor and the another gas.

A yet further aspect of the method is optimizing respective responses ofeach of two gas correlation radiometers to trace amounts of therespective target gases ethane and methane present in the freeatmosphere along the detection path to the two gas correlationradiometers. For the gas correlation radiometer for ethane detection theat least one competitive gas is a gas other than the ethane. For the gascorrelation radiometer for methane detection the at least onecompetitive gas is a gas other than the respective methane. The methodincludes a further operation of performing the initial method once withrespect to ethane as the target gas and once with respect to methane asthe target gas so that there are provided two optimized infrared filterseach having the selected center wavelength and bandwidth for use withthe respective ethane and methane gas correlation radiometers to filterlight transmitted through the free atmosphere to the respective ethaneand methane gas correlation radiometers.

A still further embodiment of the present invention includes a method ofselecting an optimum center wavelength and an optimum bandpass ofwavelengths of light to be processed by a gas correlation radiometerafter transmission of the light through the free atmosphere in whichthere may be a trace amount of a target gas and in which there is likelyto be at least one competitive other gas the presence of which in thefree atmosphere may interfere with detection of the target gas. Theoptimum center wavelength and the optimum bandpass are used inoptimizing a response of a gas correlation radiometer to the traceamount of the target gas. The method may include an operation ofdetermining a set of similarity data as a function of overlap regionswithin a spectral region. The overlap regions are for each competitivegas and the target gas and are those regions within the spectral regionin which respective absorption spectra of both the target gas and thecompetitive gas have absorption characteristics. The set of similaritydata include a plurality of data items within each of a plurality ofbandpasses, wherein one of the data items corresponds to a centerwavelength within each bandpass. The method may also include anoperation of determining a set of contrast data as a function ofnon-overlap regions within the spectral region, the non-overlap regionsbeing for each of the competitive gas and the target gas and being thoseregions within the spectral region in which the first absorptionspectrum has high absorption characteristics but the second absorptionspectrum has low absorption characteristics. The set of contrast datainclude a plurality of data items within each of a plurality ofbandpasses, wherein one of the data items corresponds to a centerwavelength within each bandpass. Optimization is completed by selectingthe center wavelength and bandpass of an infrared filter for use withthe gas correlation radiometer. Such selection is based on plotting acurve for each of various bandpasses. Each curve plots the contrast dataitem minus the similarity data item as a function of the centerwavelength. The selected center wavelength and bandpass have the highestvalue of contrast data item minus the similarity data item.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be readily understood by reference to thefollowing detailed description in conjunction with the accompanyingdrawings, in which like reference numerals designate like structuralelements.

FIG. 1A is a schematic diagram of a system of the present inventionillustrating an apparatus for detecting trace amounts of target gases,such as natural gas, remotely along a fence line detection path in thefree atmosphere;

FIG. 1B is a schematic diagram illustrating a light source provided witha modulator configured with a crystal oscillator for controlling arotating wheel that alternately blocks and passes a light beam from thesource;

FIGS. 2A and 2B depict infrared absorption spectra of a respectivetarget gas and competitive gas, illustrating overlapping lines of highinfrared absorption of both gases;

FIGS. 3A and 3B combine to define a flow chart illustrating operationsof a method of the invention for determining an optimized centralwavelength and an optimized bandpass of an IR filter for each particulartarget gas;

FIG. 4 is a graph illustrating an exemplary infrared absorption spectrumfor the target gas methane as provided by an operation of the method ofFIGS. 3A and 3B;

FIG. 5 is a graph illustrating a typical infrared transmission spectrumresulting from modeling of methane for selected parameters as providedby an operation of the method of FIGS. 3A and 3B;

FIG. 6 is a graph illustrating a typical infrared transmission spectrumdetermined for at least one competitive gas as provided by an operationof the method of FIGS. 3A and 3B;

FIG. 7 is a graph illustrating results of an operation of the method ofFIGS. 3A and 3B in which, for each of a plurality of potential IR filterbandwidths, a resultant value of the difference between contrast andsimilarity is plotted as a function of filter center wavelength;

FIG. 8 is a flow chart illustrating suboperations of an operation of themethod of FIGS. 3A and 3B for determining similarity and contrast;

FIG. 9 is a graph illustrating a sub-set of a spectral region ofinterest in which three absorption curves represent atmosphericabsorption in that spectral region, and absorption by a target gas inthat spectral region, and the product of the absorptions of theatmosphere and methane as the target gas;

FIG. 10 is a graph illustrating a sub-set of a spectral region ofinterest in which curves represent atmospheric absorption in thatspectral region, and absorption by a target gas in that spectral region,and the product of the atmospheric transmission and absorption ofmethane as the target gas;

FIG. 11 is a flow chart illustrating other aspects of the method ofoptimization of characteristics of an IR filter;

FIG. 12 is a schematic diagram of a system of the present inventionconfigured to detect and distinguish between natural gas as a target gasand other combustible gases as competitive gases;

FIG. 13 is a schematic diagram illustrating lock-in amplifier circuitryof the system;

FIG. 14 depicts a flow chart illustrating a method of processing atarget gas measurement technique, which relates to a ratio of measureddata to null data;

FIGS. 15A and 15B define suboperations of certain operations of the flowchart of FIG. 14;

FIG. 16 is a schematic diagram illustrating circuitry for accuratelycontrolling the operation of a light detector;

FIG. 17 is a schematic diagram illustrating a vehicle-mounted system fordetecting trace amounts of target gases, such as natural gas, remotelyalong a moderately long detection path in the free atmosphere;

FIG. 18 is a schematic diagram illustrating a stationary system fordetecting trace amounts of target gases, such as natural gas, remotelyalong a long detection path in the free atmosphere; and

FIG. 19 is a schematic diagram illustrating an airborne system fordetecting trace amounts of target gases, such as natural gas, remotelyalong a long detection path in the free atmosphere.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An invention is described for a method of and apparatus for detectinggases, especially trace amounts of target gases, such as natural gas,remotely along long-range paths in the free atmosphere. The exemplarynatural gas detection is preferrably enabled by simultaneously remotelydetecting methane and ethane along the same long-range path in the freeatmosphere. Details are described for systems and methods in which anactive gas correlation radiometer is configured with separate andsimultaneously operating background and gas channels. The backgroundchannel is configured with a blank cell to output data that variesaccording to whether there is target gas along the path in the freeatmosphere. The gas channel is configured with a target gas cell tooutput other data that is independent of whether there is target gasalong the path in the free atmosphere. It will be obvious, however, toone skilled in the art, that the present invention may be practicedwithout some or all of these specific details. In other instances, wellknown apparatus or process operations have not been described in detailin order not to obscure the present invention.

A system 50 of the present invention is shown in FIG. 1A for detectingthe presence of trace amounts of a specific gas 51 within the freeatmosphere 52. The specific gas 51 is one gas of many gases that mayexist in the free atmosphere 52. For purposes of description, the gasesthat may exist in the free atmosphere 52 other than the specific gas 51are referred to as background, or competitive, gases 53. In the sensethat in the system 50 the specific gas 51 is “targeted”, i.e.,specifically identified for detection, and during detection the specificgas 51 is not confused with any of the many other competitive gases 53,the specific gas 51, is referred to as the “target” gas. Similarly, inthe sense that in the system 50 there may be many specific gases 51 thatare “targeted” for detection and during detection the specific gases arenot confused with any of the many other competitive gases 53, thespecific gases 51 are referred to as the “target” gases. Referenceherein to one target gas 51 may include reference to more than onetarget gas 51 as the context permits. The potential target gases 51which may be detected by the system include, but are not limited to,CH4, NO, SO2, NO2, NH3, HNO3, OH, HF, HCl, HBr, HI, CIO, OCS, H2CO,HOCl, N2, HCN, CH3Cl, H2O2, C2H2, C2H6, and PH3. As described below inregard to FIG. 4, such target gases 51 have “rich” absorption spectra.

In the absence of the system 50, local conditions in the free atmosphere52 may render such detection of the target gases 51 difficult. Suchdifficulty may be based, for example, on the fact that the target gases51 may only be present in the free atmosphere 52 in trace amounts. Traceamounts may be in such low concentrations as about one or a few partsper billion (PPB), or in such concentrations as to approach a pure gas(˜10×10⁸ PPB). Such low concentration of one or a few PPB is the lowerend of a trace gas detection range of the system 50 with respect tocertain trace gases 51, such as ethane, for example. Such lower end isalso referred to as the “minimum detectable concentration” of the system50.

In contrast to such trace amounts, the concentration of water vapor inthe free atmosphere 53 is generally measured as 30,000 parts-per-million(PPM)-. The water vapor concentration within the free atmosphere 52 ishighly variable and is determined by the season, altitude, latitude, andlocal weather events, for example. Generally, such water vaporconcentration may vary from 1,000 to 40,000 PPM, whereas theconcentration of the target gas methane 51 is due to natural background(biological) sources and is typically in the range of from about 0.5 PPMto about 5.0 PPM within the free atmosphere 52. Since natural gas isalmost entirely comprised of methane, the methane concentration leakingfrom the pipe, at the site of the leak, approaches 8×10⁵ PPM (˜80% ofpure gas). However, the methane rapidly dilutes into the free atmosphere52, and the methane concentration rapidly approaches that of the naturalbackground methane concentration. Due to the limited sensitivitydescribed above, the Flame Ionization Detectors have a combustible gasconcentration threshold on the order to 10 to 100 PPM to indicate apotential natural gas pipeline leak. As described below, the system 50may provide a minimum detectable methane concentration of approximately50 PPB, which is a higher minimum detection concentration than forethane because of the lower depth of IR absorption of the methane IRabsorption spectrum as compared to the depth of IR absorption of theethane IR absorption spectrum.

Difficulty of such detection may also result when the target gas 51 ismixed with, or present with, the competitive gases 53. The competitivegases 53 may include water vapor and/or undesired (non-target)background gases that may be in the same free atmosphere 52 as containsthe target gas 51, for example. The competitive gases 53 are referred toas “competitive” gases because such gases 53 are not a target gas 51,yet there are overlaps 56 in absorption spectra 54 of the target gas 51and the background gases 53. FIGS. 2A and 2B depict respective exemplaryinfrared absorption spectra 54T and 54C of such respective target gas 51and competitive gases 53, wherein the overlap 56 is at an absorptionline 57 having a wavenumber at about 3015 cm⁻¹. The spectrum 54C is aportion of a spectrum for United States Standard Atmosphere at certainconditions, and the spectrum 54T is a portion of a methane spectrum, forexample.

The free atmosphere 52 shown in FIG. 1A is an open and uncontained partof the Earth's atmosphere. The free atmosphere 52 is distinguished fromcontainers (not shown) in which samples of a target gas 53 may bereceived for analysis by prior art equipment (not shown). Such targetgas 51 in the containers is not “free”. The free atmosphere 52 issubject to weather-related changes in pressure, temperature, atmosphericturbulence and variability, for example, as well as changes due toindustrial and residential events. Such events may include dischargeinto the free atmosphere 52 of target gas 51 and competitive gases 53,for example.

Light 58 transmitted by the system 50 may be directed along a detectionpath 59 through the free atmosphere 52. At any moment of time, the light58 is subjected to the current conditions of the free atmosphere 52 at aparticular place, or location, along the detection path 59. Light 58arriving at that same particular location at a next moment of time issubjected to later, then-current, conditions of the free atmosphere 52.Because the conditions of any such particular location of the freeatmosphere 52 along the detection path 59 may vary from time to time(e.g., from one tenth of a second to a next tenth of a second, forexample) the conditions of the free atmosphere 52 vary temporally, andare said to be dynamic.

FIG. 1A also shows the system 50 configured with a source 61 of thelight 58. The source 61 is active, that is, the source is configured toproduce and direct the light 58 into the free atmosphere 52. The light58 is directed along the detection path 59, which is a beam-like regionin which the target gas 51 and the competitive gases 53 may be present.In a general sense, the light 58 is broadband radiation within a rangeof wavenumbers from about 0.2 micrometers to about one hundredmicrometers. The source 61 of a fence line embodiment 50-1 (FIG. 1A) ofthe system 50, for example, may be configured to direct the light 58from a transmission location at a near end 62 of the detection path 59to a far end 63 at a receiver location spaced from the transmissionlocation. Such spacing of the ends 62 and 63 may be from about thirtyfeet to about 5300 feet, which in turn provides a selected length of thedetection path 59 as described below with respect to various embodiments50-1 through 50-5 of the system 50.

At the transmission location, the source 61 may be provided with amodulator 64 and beam forming optics 66, to provide the light 58 in theform of a collimated modulated light beam 67. The collimated modulatedlight beam 67 is thus generated at the source 61 and is transmitted fromthe near end 62 along the detection path 59 through the free atmosphere52 to the far end 63. In an embodiment 50-5 described below in whichmodulation of the light beam 67 is not required, the modulator 64 is notpart of the source 61. The light beam 67 is transmitted along thedetection path 59 through any competitive gas 53 and through any targetgas 51. During such transmission, depending on the atmosphericconditions along the detection path 59, the light 61 may be absorbed,scattered, or reflected, and such, absorption, scattering, andreflection may vary with location along the detection path 59, or withtime, or with respect to the length of the detection path 59, forexample. While such scattering or reflection may divert the light beam67 from the detection path 59, such absorption may result from infraredabsorption typified by FIGS. 2A and 2B, for example. Such absorption,scattering, and reflection reduce the intensity of the light beam 67.

FIG. 1A shows the light beam 67 at the far end 63 of the detection path59 entering a receiver 68 of the system 50. The receiver 68 isconfigured with a receiving telescope 69 that focuses the light beam 67for transmission through an infrared (IR) filter 71. The IR filter 71 isconfigured with an optimized central wavelength and an optimizedbandpass as described below to provide substantially increasedsensitivity to a specific one of the target gases 51 and substantiallyincreased selectivity of that target gas 51 to avoid erroneous detectionof any competitive gas 53 as that target gas 51. The receiver 68 isconfigured so that the IR filter 71 transmits the light 58 having thenow-optimized central wavelength and the now-optimized bandpass to abeam splitter 72. The beam splitter 72 is configured to provide twosplit light beams 73 that are transmitted simultaneously to, andsimultaneously enter, a dual-channel gas correlation radiometer (GCR)subsystem 74.

The GCR subsystem 74 is configured with two separate channels 76 and 78that simultaneously receive the respective split light beams 73 havingthe optimized central wavelength and the optimized bandpass. A first ofthe beams 73-1 is transmitted through the channel 76, which isconfigured with a neutral density filter 79 and a blank cell 81. Thefirst channel 76 is configured with a first detector 82 which outputsblank channel data 83 that varies according to whether there is targetgas 51 in the detection path 59 through which the light beam 67 wastransmitted on the way to the receiver 68. A second of the beams 73-2 istransmitted through the channel 78, which is configured with a targetgas cell 84. The second channel 78 is configured with a second detector86 which outputs target gas channel data 87 that is independent ofwhether there is target gas 51 in the region, i.e. along the detectionpath 59 through which the light beam 67 was transmitted on the way tothe receiver 68. The blank channel data 83 and the target channel data87 are supplied to a lock-in amplifier 88, the output of which isapplied to a display and data processor 89. The splitting of the lightbeam 67 in this manner results in the simultaneous reception by theprocessor 89 of the data 83 and 87 for simultaneous processing.

Because of the modulation at the source 61, the modulated collimatedlight beam 67 has been subjected to the atmospheric conditions along thedetection path 59 at successive moments of time (see successively latertimes t1, t2, and t3, for example, in FIG. 1B) during which the lightbeam 67 is transmitted from the near end 62 to the far end 63 of thedetection path 59. Such atmospheric conditions further modulate thelight beam 67, but this further atmospheric modulation is different fromthe modulation by the modulator 64 at the source 61. The lock-inamplifier 88 selects from this combination of modulated light 58 onlythe light 58 having the modulation imposed by the modulator 64 at thesource 61, such that the data 83 and 87 do not include the effects ofatmospheric turbulence, jitter, beam wander, and changes in the index ofrefraction, for example. Thus, each of the data 83 and 87 received bythe processor 89 at any moment of time represents the influence of thesame current atmospheric conditions of the free atmosphere 52 as thelight 58 is transmitted along the detection path 59, as may have beenmodified only by transmission through the respective channels 76 and 78.As described above, the first detector 82 outputs the blank channel data83 that varies according to whether there is target gas 51 in thedetection path 59 through which the light beam 67 was transmitted on theway to the receiver 68. The second detector 86 outputs target gaschannel data 87 that is independent of whether there is target gas 51 inthe region, i.e. along the detection path 59 through which the lightbeam 67 was transmitted on the way to the receiver 68. The channels 76and 78 thus modify the light beam 67 so that together the data 83 and 87represent various factors. The factors include whether or not there wastarget gas 51 along the detection path 59 during the successive momentsof time t1, t2, t3, etc. during which the light beam 67 was transmittedfrom the near end 62 to the far end 63 along the detection path 59, andif target gas 51 was detected, the concentration of the target gas 51.

Considering the structure and operation of the system 50 shown in FIG.1A in more detail, the source 61 is configured with a broadband lightsource 90 that may output the broadband light 58 described above. Thelight source 90 may be a thermal lamp configured for high infraredoutput. A suitable lamp is a blackbody source having an output spectrumthat follows the universal blackbody relationship in which the intensityof the light 58 output is proportional to T_(color), the colortemperature of the lamp. For example, the following may be used for thelight source 90: silicon carbide lamps (having a T_(color) of ˜1000Kelvin (K)), halogen lamps (having a T_(color) of ˜3000 K), Xe lamps(having a T_(color) of ˜6000 K), tungsten lamps (having a T_(color) of˜5000 K), and plasma glow lamps (having an output equivalent to the Xelamp). The Xe lamp, for example, can achieve a total optical poweroutput of about seventy-five Watts. The plasma glow lamp may be aglowing one millimeter (mm) plasma sphere, mounted at the focal point ofan elliptical reflective mirror. The plasma glow lamp has a surfaceemission temperature of 6000 K, a total optical output of 75 watts, andan output of ˜6 milliwatts within the typical bandpass of the IR filter71. Another blackbody source suitable for use as the light source 90 isan arc lamp, which may be physically smaller and more-efficient than theglowing plasma sphere. The arc lamp produces about three milliwatts inthe optical bandwidth (about 2800 to 3200 cm⁻¹) required for detectionof methane and ethane as the target gases 51. As the length of thedetection path 59 increases, it is preferable to use the source 90having the higher T_(color).

Suitable broadband sources 61 must be capable of transmitting thegenerated light 58 into the free atmosphere 52. In order to assure thatthe source-generated light 58 is transmitted into the free atmosphere52, the broadband source 61 is not encased in outer bulbs, and does nothave other optical elements (collectively “optical components”), that donot transmit well at the wavelengths of interest for the detection ofthe target gas(es) 51. When the target gas 51 is either methane orethane, for example, quartz, germanium, and silicon optical componentsare not used since they do not provide for the optimal transmission ofthe source-generated light 58 into the free atmosphere 52. In apreferred embodiment of the source 61, potassium bromide may be used forthe optical components since it has adequate optical transmission.However, potassium bromide scratches easily. In a more preferredembodiment of the source 61, sapphire is used for the optical componentsbecause sapphire transmits well (>90%) in a range from the visiblewavelengths to approximately 7 μm and is more durable than potassiumbromide.

In review, the wavelength of the light 58 output from the source 90 maybe broadband, preferably in the range of about 0.2 micrometers to aboutone hundred micrometers, for example. More preferably, the wavelength ofthe light 58 output from the source 90 may be in the range of about 3micrometers to about 10 micrometers. Most preferably, the wavelength ofthe light 58 output from the source 90 is in the range of about 3.12micrometers to about 3.57 micrometers. Each such range of wavelengthsmay be referred to as being “broadband” in the sense that these rangesare substantially broader than the narrow emission spectra used in theTDLAS, laser absorption spectroscopy and DIAL instruments describedabove.

The fence line embodiment 50-1 of the system 50 shown in FIG. 1A may beprovided with a transmitter housing 91 at the transmitter locationadjacent to the near end 62 of the detection path 59. The fence lineembodiment 50-1 may also be provided with a receiver housing 92 at thereceiver location adjacent to the far end 63 of the detection path 59.When the detection path 59 is in a range of lengths from about thirtyfeet to about 100 feet, the modulator 64 may be hard-wired to thelock-in amplifier 88 for directly transmitting to the amplifier 88 ademodulator signal (not shown) that is in-phase and in-frequency withthe modulating frequency of the modulator 64. However, when thedetection path 59 is in a range of lengths from about 100 feet to about5,300 feet, the modulator 64 cannot be hard-wired to the receiver 68. Inthis situation, as described below, based on receipt of data (orsignals) derived from the source-modulated split light beams 73 thelock-in amplifier 88 determines the frequency and phase of the receivedsplit light beams 73 and locks to such frequency and phase.

In one embodiment of the system 50 the source 61 may be configured withthe modulator 64 for amplitude modulation, such as by use of a chopperhaving sequentially positioned openings, or an electromagneticallydriven tuning fork, for example. FIG. 1B shows a preferred embodiment inwhich the source 61 may be provided with the modulator 64 configuredwith an internal crystal oscillator 93 and motor control circuit 93-1 toprovide closed-loop control of the modulation frequency. A rotatingwheel 94 alternately blocks and passes the light beam 67. In thisembodiment, the user selects the desired modulation rate through anexternal input 93-2. The motor control circuit 93-1 tunes the output ofthe internal oscillator 93 to this value. The modulator 64 includes anoptical pick-off 95 associated with the wheel 94 for providing an outputsignal 96-1 that is provided to the motor control circuit 93-1. Themotor control circuit 93-1 compares the optical pick-off output 96-1with the output from the crystal oscillator 93, which was previouslytuned to the user input 93-2. Then, the motor control circuit 93-1either speeds-up or slows-down the rotation of the wheel 94 by providingappropriate drive signals 96-2 to a chopper motor 98. In this manner,the optical modulation frequency is precisely controlled to auser-selectable value using the precision crystal oscillator 93. Inaddition, the motor control circuit 93-1 provides a reference signal 97which is precisely in phase with, and at the same frequency as, theoptical modulation frequency. This reference signal 97 may be providedto the lock-in amplifier circuit 88 (FIG. 13). Using this arrangement,the optical modulator 64 provides a precision, selectable chopping ormodulation, frequency, which preferably is 1 kHz ±0.01 Hz, or 10parts-per-million. In a more general sense, the modulation frequency iswell above the 1/f noise of the detectors 82 and 86.

In another embodiment, the source 61 may be configured with themodulator 64 for frequency modulation, such as by use of a pressure cellthrough which the light 58 from the source 90 is transmitted.

FIG. 1A shows the beam forming optics 66, which may be an amateur-gradetelescope having total reflectivity. Such telescopes are configured withtotally reflective optics and an f-number of 4 (f/4). Such telescopeshave the ability to direct the light beam 67 along a 5300 foot detectionpath 59 with a divergence of a few milliradians, so that if the lightbeam 67 is eight inches in diameter at the near end 62 the light beam 67may have a diameter of approximately sixty inches at the far end 63 ofthe 5300 foot path 59. One advantage of the system 50 is that suchtelescope 66 having the totally reflective optics for use in the IRregion is very low cost as compared to beam forming optics constructedfrom refractive optics. Such optics 66 may be supplied by Orion, SantaCruz, Calif.; or Meade of Irvine, Calif., or Celestron International ofTorrance, Calif., for example. The receiving telescope 69 may also be anamateur-grade telescope similar to the optics 66, having the ability toreceive a portion of the light beam 67 having the diameter ofapproximately 60 inches at the end 63 of the 5300 foot path 59.

The IR filter 71 is configured (e.g., optimized) for each differenttarget gas 51, such that the optimized central wavelength and theoptimized bandpass provide substantially increased sensitivity to theparticular target gas 51 and substantially increased selectivity of suchtarget gas 51 to avoid erroneous detection of any competitive gas 53 asthe target gas 51. In a preferred embodiment of the IR filter 71, the IRfilter 71 is configured to respond to light wavelengths within a bandcorresponding to strong IR absorption by the specific target gas 51. Theband also corresponds to weak IR absorption by gases 53 other than thespecific target gas present 51 in the free atmosphere 52. The word“weak” is relative to the stronger IR absorption by the specific targetgas 51. It may be said that such band provides a high degree ofdissimilarity in the infrared absorption spectra of the respectivetarget gas 51 and competitive gas 53.

A method of the invention is used to determine the optimized centralwavelength and the optimized bandpass for each particular target gas 51.FIGS. 3A and 3B show a flow chart 101 of the method. The method maystart by moving to an operation 102 in which a modeled spectrum of thetarget gas 51 is determined. A typical initial spectrum that may beexamined in operation 102 is the exemplary infrared (IR) absorptionspectrum 103 shown in FIG. 4, which is for methane, and extends fromabout wavenumber 3600 cm⁻¹ to about wavenumber 700 cm⁻¹. This exemplaryspectrum 103 may be obtained from the spectral database “InfraredSpectra For Quantitative Analysis of Gases” by P. L. Hanst and S. T.Hanst, Infrared Analysis Inc., Anaheim, Calif., for example. The targetgas 51 preferably has a spectrum 103 that is “rich”, in that there arestrong absorption lines 104 in a wide range 106 of wavenumbers (e.g.,from about 3200 to about 1300 cm⁻¹). The target gases 51 listed aboveare examples of target gases 51 having rich absorption spectra.

The determination of the exemplary target gas IR spectrum 103 inoperation 102 may also include modeling the absorption features for thetarget gas cell 84. Such modeling may be performed using a suitableline-by-line database, such as the U.S. Air Force Research Laboratorydatabase High Transmission Database, U.S. AFRL, PL/GPOS, Hanscom AFB,Mass. (known as “HITRAN”). The spectral absorption features for aspecific optical geometry may be obtained by use of computer softwaresuch as the U.S. AFRL FASCODE (“Fast Atmospheric Signature Code”), U.S.AFRL, PL/GPOS, Hanscom AFB, Mass.; or a PCLNWIN program made by OntarCorporation, North Andover, Mass., or the “HITRANPC” program of theUniversity of South Florida Preferably, the parameters used in suchmodeling should generally be those to be experienced in the freeatmosphere 52 in actual field use of the system 50. More preferably, theparameters should be those reasonably expected to be present in the freeatmosphere 52 in actual field use of the system 50. Most preferably, theparameters used in such modeling should be those known to exist mostfrequently in the particular location of the free atmosphere 52 in whichthe system 50 is to be used. This modeling provides, or tailors, the IRabsorption spectrum (e.g., the spectrum 103 of FIG. 4) for suchparameters, which may include the concentration, pressure andtemperature of the target gas 51, and the pathlength through the opticalgeometry. For example, the wavelength, breadth and transmissioncharacteristics of the target gas 51 change with changes in suchparameters. FIG. 5 shows a typical transmission spectrum 107 resultingfrom such modeling of methane for the parameters one centimeter (cm)pathlength, 288.2 K methane temperature, and one atmosphere methanepressure. The exemplary temperature and pressure parameters are the sameas the U.S. Standard Atmosphere model at sea level (see Anderson, G. P,et al., Report AFGL Atmospheric Constituent Profiles, 0-120 km, ReportNo. AFGL TR-86-0110, Air Force Geophysics Laboratory, Hanscom AFB,Mass., 1986, for example).

The exemplary modeled absorption spectrum 107 shown in FIG. 5 includesmany deep, or highly absorbing, IR absorption lines 108D. It may beunderstood that FIG. 5 represents, for methane as an exemplary targetgas 51, the results of the operation 102, which identifies a spectralregion of a first absorption spectrum of the target gas 51. The spectralregion is the wavenumber region from 2800 to 3200 cm⁻¹, and the spectrumcorresponds to the above-described selected parameters of target gasconcentration, target gas temperature, target gas pressure, and pathlength through the target gas 51. The spectral region has the pluralityof high (or deep) absorption lines 108D and low absorption lines 108W.In summary, operation 102 (FIG. 3A) involves identifying a firstabsorption spectrum of the target gas 51 corresponding to selected gasconcentration, temperature, pressure, and path length.

The method of flow chart 101 moves to operation 109 in which anabsorption spectrum 110 (e.g., see FIG. 6) is determined for at leastone competitive gas 53 that is, and preferably for all typicalcompetitive gases 53 that are, expected to be in the free atmosphere 52in the detection path 59 in the use of the system 50. Considering thespectrum 110 as corresponding to all such gases 53, the spectrum 110 maybe that obtained from a suitable line-by-line database, such as theabove-identified HITRAN database. The spectral absorption features for aspecific optical geometry of all of the competitive gases 53 may beobtained by use of computer software, such as one of the aboveidentified FASCODE, PCLNWIN or HITRANPC programs, for example. The orderof preference of the parameters used in such modeling are as describedabove for the target gas 51. This modeling also tailors the combinedabsorption spectra of the competitive gases 53 for such parameters,which may include the concentration, pressure and temperature of thecompetitive gases 53, and the pathlength through the optical geometry.For example, the wavelength, breadth and transmission characteristics ofthe competitive gases 53 change with changes in such parameters. FIG. 6shows a typical absorption spectrum resulting from such modeling of thecompetitive gases 53 in the U.S. Standard Atmosphere, modeled at sealevel conditions for the parameters of: 23 km visibility aerosols, onekm pathlength, 288.2 K atmospheric temperature, and one atmosphere gaspressure. The exemplary modeled absorption spectrum 110 shown in FIG. 6also includes many deep, or highly absorbing, absorption lines 117D, andmany weak absorbing lines 117W. The modeling represented by FIG. 6includes competitive gases 53 such as H2O, O2, O3, N2O, CO, CO2, andCH4. Also, the U.S. Standard Atmosphere includes possible target gases51 such as NO, SO2, NO2, NH3, HNO3, OH, HF, HCl, HBr, HI, CIO, OCS,H2CO, HOCl, N2, HCN, CH3Cl, H2O2, C2H2, C2H6, CH4 and PH3.

It may be understood that FIG. 6 represents, for all of the competitivegases 53 as exemplary competitive gases 53, the results of operation109. FIG. 6 is for the spectral region from wavenumber 2800 to 3200cm⁻¹, and provides the absorption spectrum 110 corresponding to theabove-described parameters. The spectrum 110 includes the non-absorbingregions (the lines 117W) corresponding to the low-absorptioncharacteristics (lines 108W, FIG. 3A) of the exemplary methaneabsorption spectrum 107, and the highly absorbing lines 117Dcorresponding to deep-absorption characteristics (lines 108D) of suchabsorption spectrum 107. In summary, operation 109 involves identifyinga second absorption spectrum of each of at least one other competitivegas 53. The second absorption spectrum corresponds to the selected gasconcentration, temperature, pressure, and path length.

Having the absorption spectra 107 and 110, a preferred embodiment of theinvention may compare the IR absorption spectrum 107 to the IRabsorption spectrum 110 to identify one wavelength range in which thereis a high degree of dissimilarity in the IR absorption spectra. Thedissimilarity is indicated by wavelengths in the range at which there ishigh IR absorption by the target gas 51 and low IR absorption by the atleast one competitive gas 53.

A more preferred embodiment of the invention configures the IR filter 71with a band pass that is optimal for detecting the trace amounts of thetarget gas 51. The optimal band pass includes infrared absorptionfeatures of the target gas 51 to be detected and a suitable transmissionregion of all common gaseous constituents of the free atmosphere 52other than the target gas 51. Within the band pass there is a relativelysmall degree of spectral overlap between the IR absorption features ofthe target gas IR absorption spectrum 107 and that of the atmospheric IRabsorption spectrum 110.

In a most preferred embodiment of the invention, the method moves tooperation 118, which is a further part of determining optimalcharacteristics for possible IR filters 71. In a general sense,operation 118 involves determining both “similarity” and “contrast” thatexist between the respective absorption spectra 107 and 110 of thetarget gas 51 (the IR filter 71) and that of the atmosphere (i.e., thecompetitive gases 53). Once values of contrast and similarity aredetermined, the method moves to an operation 119 in which the differencebetween the respective values of contrast and similarity is determinedfor several center wavelengths and bandpasses of possible IR filters 71.

The method moves to an operation 121 (FIG. 3B) in which, for eachpotential IR filter bandpass, the resultant value of the differencebetween the contrast and similarity is plotted as a function of filtercenter wavelength (see FIG. 7). Each filter bandpass is plotted as aseparate data set, that is, as a separate curve 122 in FIG. 7. Fiveexemplary curves 122-1 through 122-5 are shown in FIG. 7, correspondingto five bandwidths identified respectively as 0.025 microns Half Widthat Half Maximum (“HWHM”), 0.05 microns HWHM, 0.075 microns HWHM, 0.1microns HWHM, and 0.125 microns HWHM.

The method moves to an operation 123 in which there is a selection ofthe combination of filter center wavelength and bandpass that results inthe largest value of contrast minus similarity. Such selection providesthe optimal IR filter characteristics for the specific trace gas 51,which is methane in the exemplary situation of FIG. 5 which shows thespectrum 107. The curves 122-1, 122-2, and 122-3, for example, providechoices for use in such selection. All curves 122-1 through 122-5indicate that infrared filters 71 with a center wavelength from 3.2 toapproximately 3.37 μm would not be selected due to poor performance, asindicated by the negative values of contrast minus similarity. That is,the target gas spectrum 107 of FIG. 5 is very similar to the spectrum110 of competitive gases 53 shown in FIG. 6 within the spectral regionbetween 3.2 to about 3.37 μm. For infrared filters 71 having a centerwavelength between about 3.37 to 3.5 μm, all curves 122-1 through 122-5have positive values of contrast minus similarity, indicating animproved degree of contrast between the target gas spectrum 107 shown inFIG. 5 and the spectrum 110 of competitive gases 53 (shown in FIG. 6).Within this improved region from 3.37 to 3.5 μm, curve 122-5(corresponding to a filter bandpass of 0.125 μm HWHM) provides thepoorest system performance since it corresponds to the smallest value ofcontrast minus similarity. Alternately, curve 122-3 achieves a higherpeak value of contrast minus similarity, at the center wavelength ofapproximately 3.45 μm. Also, two filter bandpasses (curves 122-2 and122-3) result in equivalent values of contrast minus similarity (seeHWHM of respective 0.05 μm and 0.075 μm). Therefore, these two infraredfilters 71 corresponding to curves 122-2 and 122-3 will provideequivalent performance for detecting only the target gas 51 of interest(methane in this example) while rejecting light 58 having wavelengthscorresponding to the strong absorption bands of all competitive gases53. Since the filter bandwidth of curve 122-3 is wider (0.075 μm HWHM),an IR filter 71 having such center wavelength (about 3.45 μm) andbandwidth (0.075 μm HWHM) will transmit a larger signal amplitude of thelight 67 to the detectors 82 and 86 than will an IR filter 71 based oncurve 122-2 (0.05 μm HWHM).

Having the curves 122-1 through 122-5, for example, certain additionalconsiderations may be reviewed in selecting the optimal centerwavelength and optimal bandpass, such as manufacturability of the IRfilter 71. An IR filter 71 having a very narrow center wavelength (e.g.,less than 0.5 μm) in the visible range (300 to 800 nm) may bemanufactured more easily and at less cost than such IR filter 71 in theinfrared or ultraviolet range. Also, consideration may be given to thebreadth of the optimal bandpass, which may influence manufacturabilityand cost to manufacture, especially with respect to the particularcenter wavelength of the IR filter 71.

More preferably, the selection of the optimal center wavelength andoptimal bandwidth may be made according to which combination of centerwavelength and bandpass results in the largest value of contrast minussimilarity for the trace gas 51. For example, a center wavelength ofapproximately 3.45 μm and the either the 0.05 or 0.075 μm bandpass ofcurves 122-2 or 122-3, respectively, may be considered as the optimalcenter wavelength and optimal bandpass. Having made the selection of theoptimal center wavelength and optimal bandpass, the method is DONE.

In view of this method, for example, when the GCR subsystem 74 isconfigured with the IR filter 71 to respond to light 67 transmittedthrough the detection path 59, the subsystem 74 (with the IR filter 71)is configured to respond to light wavelengths within a band in whichthere is a high degree of dissimilarity in the atmospheric infraredabsorption spectra. The dissimilarity is indicated by wavelengths atwhich there is high (or strong) infrared absorption by the target gas 51and low (or weak) infrared absorption by the competitive gas 53.

FIG. 8 shows in more detail suboperations of operation 118, whichinvolves a suboperation 126 for determining similarity, which isdefined, in general, as those spectral regions where both the target gas51 (represented by the spectrum 107, FIG. 5) and the atmosphere(represented by the spectrum 110, FIG. 6), possess the overlappingabsorption features, i.e., the absorption lines 108D and 117D.

In more detail, suboperation 126 may involve determining a set ofsimilarity data as a function of overlap regions 56 (FIGS. 2A and 2B)within the spectral regions shown in FIGS. 2A and 2B (see absorptionspectra 54C and 54T). In FIGS. 5 and 6 the overlap regions maycorrespond to highly absorbing IR absorption lines 108D and 117D foreach of respective competitive gas 53 and target gas 51, and are thoseregions within the spectral region in which the respective IR absorptionspectra 107 and 110 of both the target gas 51 and at least one othercompetitive gas 53 have absorption characteristics. The set ofsimilarity data includes a plurality of data items (i.e., points on suchgraphs) within each of a plurality of bandpasses that include suchhighly absorbing lines 108D and 117D. One of the data items correspondsto the center wavelength within each bandpass.

In still more detail, a similarity determination involves calculatingthe product of the atmospheric IR absorption (1 minus transmission T)and the target gas cell IR absorption on a wavelength-by-wavelengthbasis. That is, a change (delta) in similarity is determined by:

∂similarity=T _(filter)(1−T _(atmosphere))(1−T _(cell))∂λ,  Equation (3)

where T_(atmosphere) is the wavelength-dependent optical transmissionthrough the atmosphere (such as shown in FIG. 6), T_(cell) is thewavelength-dependent optical transmission through the target gas channel78 of the GCR subsystem 74 (such as that shown in FIG. 5 for the case ofmethane), T_(filter) is the wavelength-dependent optical transmissionthrough the IR filter 71, and delta lambda is an increment of wavelengthwithin the bandwidth of the IR filter 71.

An exemplary first operation in the calculation of Equation 3 is toperform a product of absorptions, as follows:

Product=(1−T_(atmosphere))(1−T_(cell)).  Equation (4)

FIG. 9 illustrates a sub-set of the spectral region of interest (e.g.,shows wavenumbers 2880 through 2900 cm⁻¹) and shows three absorptioncurves 131, 133, and 135. Curve 131 represents the atmosphericabsorption in that spectral region, curve 133 represents the absorptionby the target gas 51 in that spectral region, and curve 135 representsthe product of the absorptions of curves 131 and 133 for methane as thetarget gas 51. FIG. 9 shows the spectral regions where the spectrum 110of the competitive gas 53 (the atmosphere in this case) and the spectrum107 of the target gas 51 (methane in this example) are similar, or wellcorrelated. A high degree of similarity between these spectra results ina GCR that cannot distinguish between the competitive gas species andthe target gas of interest.

To obtain similarity, in operation 126 Equation 3 is computed asfollows: $\begin{matrix}{{{similarity} \equiv {\int_{\lambda_{1}}^{\lambda_{2}}{{T_{filter}\left( {1 - T_{atmosphere}} \right)}\left( {1 - T_{cell}} \right){\partial\lambda}}}},} & {{Equation}\quad (5)}\end{matrix}$

where λ₁ and λ₂ denote the bandwidth B of the IR filter 71. Operation126 multiplies the absorption product by the filter transmission profileT_(filter) and an integration is performed over the filter bandwidth λ₂to λ₁.

Still referring to FIG. 8, upon conclusion of the suboperation 126 ofoperation 118, the method moves to a suboperation 137 for thedetermination of contrast. Contrast is defined, in general, as thosespectral regions where the target gas 51 (represented by the spectrum107, FIG. 5) has deep absorption features 108D, and the atmosphere(represented by the spectrum 110, FIG. 6), possesses the weak absorptionfeatures, i.e., the absorption lines 117W. Thus, the determination ofspectral contrast is based on the regions of non-overlapping absorptionbetween the atmospheric spectrum 110 and the spectrum 107 of the targetgas 51.

In a general sense, operation 137 may involve determining a set ofcontrast data as a function of respective non-overlap regions 108D and117W within the spectral regions of respective FIGS. 5 and 6. Thenon-overlap regions 117W and 108D are for each of the respectivecompetitive gas 53 and target gas 51 and are those regions within thespectral region in which the target gas absorption spectrum 107 has thehigh absorption characteristics (lines 108D) but the competitive gasabsorption spectrum 110 has the low absorption characteristics (117W).The set of contrast data includes a data item corresponding to each ofmany possible the bandpasses and center wavelengths of the infraredfilter 71.

In more detail, the contrast determination of suboperation 137 involvescalculating the product of the atmospheric transmission T and the targetgas cell absorption on a wavelength-by-wavelength basis. That is, achange (delta) in contrast is determined by:

∂contrast=T _(filter)(T _(atmosphere))(1−T_(cell))∂λ.  Equation (6)

An exemplary first operation in the calculation of Equation 6 is toperform another product of atmospheric transmission and target gasabsorption per Equation (7):

Product=(T_(atmosphere))(1−T_(cell);)  Equation (7):

where: (T_(atmosphere))=atmospheric transmission and (1−T_(cell))=targetgas absorption. FIG. 10 illustrates a sub-set of the spectral region ofinterest (e.g., shows wavenumbers 2880 through 2900 cm⁻¹) and showsthree absorption curves 138, 139, and 141. Curve 138 represents theatmospheric transmission in that region, curve 139 represents theabsorption by the target gas 51 (methane) in that region, and curve 141represents the product of the respective transmission of curve 138 andabsorption of curve 139 for methane as the target gas 51. FIG. 10 showsthe spectral regions where the spectrum 110 of the competitive gas 53(the atmosphere in this case) and the spectrum 107 of the target gas 53(methane in this example) are dissimilar, or not well correlated. A highdegree of dissimilarity (or contrast) between these respective spectra107 and 110 results in operation of the GCR subsystem 74 thateffectively distinguishes between the competitive gas 53 and the targetgas 51 of interest.

To obtain a value of contrast, in operation 137 Equation 6 is computed:$\begin{matrix}{{contrast} \equiv {\int_{\lambda_{1}}^{\lambda_{2}}{{T_{filter}\left( T_{atmosphere} \right)}\left( {1 - T_{cell}} \right){{\partial\lambda}.}}}} & {{Equation}\quad (8)}\end{matrix}$

Having obtained the value of contrast in suboperation 137, the methodmoves to operation 119 (FIG. 3A).

In summary, referring to FIG. 11, a method of optimization of thecharacteristics of the IR filter 71 may be described by a flow chart 146in which an operation 147 selects the series of IR filter centerwavelengths for evaluation (described in more detail in reference tooperations 102 and 109 of FIG. 3A). The method moves to operation 148for determining the similarity for each such IR filter center wavelengthand bandpass selected in operations 146, as described in operation 118(FIG. 3A) and 126 (FIG. 8). The method moves to operation 149 fordetermining the contrast for each such IR filter center wavelength andbandpass selected in operation 147, as described in operations 118, and137 (FIG. 8). The method moves to operation 151 for determining thevalue of contrast minus similarity for each of the IR filter centerwavelength and bandpass options identified in operation 147, asdescribed with respect to operation 119. The method moves to operation152 in which the value of contrast minus similarity is plotted for eachof the filter center wavelength and bandwidth options identified inoperation 147, the plotting being as a function of filter centerwavelength. The plotting provides a separate one of the curves 122 foreach IR filter bandpass. The method moves to operation 153 foridentifying the optimal center wavelength and optimal bandpass thatprovides the largest value of contrast minus similarity for the tracegas 51, as described with respect to operation 123 (FIG. 3B).

The optimal center wavelength and optimal bandpass are used to providethe IR filter 71 in the system 50 to optimize the response of the GCRsubsystem 74 to a trace amount of a target gas 71 that may be present inthe free atmosphere 52 along the detection path 59 to the receiver 68.The detection path 59 may also contain the competitive gas 53 thepresence of which in the free atmosphere 52 may interfere with detectionof the trace amount of the target gas 51. Based on the IR filter 71 usedwith the GCR subsystem 74, the GCR subsystem 74 is configured to respondto the light 58 transmitted along the detection path 59 and having awavelength within the optimal bandpass. The optimal bandpass generallycorresponds to strong absorption by the at least one target gas 51 andcorresponds to weak absorption by the gases 53 present in the freeatmosphere 52 as compared to the strong IR absorption by the target gas51.

In one embodiment of the system 50, an exemplary IR filter 71 of thereceiver 68 may be provided with respect to one competitive gas 53, suchas water vapor which is a widely occurring gas. In this case, theabsorption spectrum 110 may correspond to water vapor. In anotherembodiment of the system 50, another exemplary IR filter 71 of thereceiver 68 may be provided with respect to competitive gas 53 in theform of water vapor and another non-target gas 53. In this case, theabsorption spectrum 110 may correspond to water vapor and the other gas53.

FIG. 12 shows another embodiment of the system 50 which is configured todetect and distinguish between natural gas as a target gas 51, and othercombustible gases as competitive gases 53. As described above, the mainconstituents of natural gas are methane and ethane. This embodiment ofthe system 50 may be configured as an optimal natural gas detector bybeing configured to simultaneously detect both methane and ethane toassure that the detected gas is from a natural gas leak and not from anatural emission source. FIG. 12 shows the receiver 68 portion of thesystem 50 as a methane/ethane receiver 68ME provided with the receivingtelescope 69 which directs a portion of the light beam 67 from the farend 63 to a second beam splitter 161. The light beam 67 is divided intotwo separate and simultaneous beams 162 and 163 which are respectivelydirected through two separate IR filters 71M and 71E.

An embodiment of the IR filter 71M may be configured to transmit light58 from the light beam 67 that was transmitted along the detection path59 and having a wavenumber within a band taken from the group consistingof: from about 1200 to about 1400 cm⁻¹, from about 2800 to about 3200cm⁻¹, from about 2850 to about 3000 cm⁻¹, and from about 3000 to about3200 cm⁻¹ These wavenumber bands are preferred wavenumber bands formethane as a target gas 51. More preferrably, an embodiment of the IRfilter 71M may be configured to transmit light 58 from the light beam 67that was transmitted along the detection path 59 and having a wavenumberwithin a band taken from the group consisting of: from about 2850 toabout 3000 cm⁻¹, and from about 3000 to about 3200 cm⁻¹. Thesewavenumber bands are more preferred wavenumber bands for methane as atarget gas 51. Most preferrably, an embodiment of the IR filter 71M maybe configured in the manner described above with respect to FIGS. 3A and3B through 11 with a methane-optimal central wavelength and amethane-optimal bandpass to provide substantially increased sensitivityto methane as the target gas 51 and substantially increased selectivityof methane to avoid erroneous detection of any competitive gas 53 asmethane. The particular methane-optimal center wavelength andmethane-optimal bandpass resulting from use of the methods describedwith respect to FIGS. 3A and 3B through 11 will depend on the fieldconditions described above, for example.

An embodiment of the IR filter 71E may be configured to transmit light58 from the light beam 67 that was transmitted along the detection path59 and having a wavenumber within a band of from about 2970 to 3005cm⁻¹. More preferrably, an embodiment of the IR filter 71E may beconfigured with an ethane-optimized central wavelength and anethane-optimized bandpass in such manner described above to providesubstantially increased sensitivity to ethane as the target gas 51 andsubstantially increased selectivity of ethane to avoid erroneousdetection of any competitive gas 53 as ethane. Most preferrably, anembodiment of the IR filter 71M may be configured in the mannerdescribed above with respect to FIGS. 3A and 3B through 11 with anethane-optimal central wavelength and an ethane-optimal bandpass toprovide substantially increased sensitivity to ethane as the target gas51 and substantially increased selectivity of ethane to avoid erroneousdetection of any competitive gas 53 as ethane. The particularethane-optimal center wavelength and optimal ethane-bandpass resultingfrom use of the methods described with respect to FIGS. 3A and 3Bthrough 11 will depend on the field conditions described above, forexample.

The receiver 68ME is configured so that the IR filter 71M transmits thelight beam 162 having, for example, the methane-optimal centralwavelength and the methane-optimal bandpass. The light beam 162 istransmitted to a methane-ethane embodiment of the beam splitter 72. Thebeam splitter 72 is configured in two parts 72-1 and 72-2. Each partprovides two split light beams 73, including one set of split beams 73-1and 73-2, and another set of split light beams 73-3 and 734, that aretransmitted respectively and simultaneously to, and simultaneouslyenter, one of two of the dual channel gas correlation radiometer (GCR)subsystems 74. One subsystem 74M for methane detection receives thebeams 73-1 and 73-2; and one subsystem 74E for ethane detection receivesthe beams 73-3 and 73-4. Each respective subsystem 74M and 74E isconfigured in the manner described above with respect to FIG. 1A withtwo separate channels 76 and 78 that simultaneously receive therespective split light beams 73. Referring to FIG. 1A for the details ofthe respective subsystems 74M and 74E, the first channel 76 of themethane subsystem 74M is configured with the first detector 82 thatoutputs blank channel data 83 (referred to here as 83M, for methane)that varies according to whether there is methane 51 in the detectionpath 59 through which the light beam 67 was transmitted on the way tothe receiver 68ME. A second of the beams 73-2 is transmitted through thesecond channel 78 of the methane subsystem 74M, which is configured witha methane target gas cell 84. The second channel 78 is configured withthe second detector 86 which outputs target gas channel data 87(referred to here as 87M, for methane) that is independent of whetherthere is methane target gas 51 in the region, i.e. along the detectionpath 59 through which the light beam 67 was transmitted on the way tothe receiver 68ME. The blank channel data 83M and the target channeldata 87M are supplied from the methane subsystem 74M to the lock-inamplifier 88, the output of which is applied to the display and dataprocessor 89.

The first channel 76 of the ethane subsystem 74E is configured with afirst detector 82 that outputs blank channel data 83 (referred to hereas 83E, for ethane) that varies according to whether there is ethane 51in the detection path 59 through which the light beam 67 was transmittedon the way to the receiver 68. The second channel 78 of the ethanesubsystem 74E is configured with a second detector 86 which outputstarget gas channel data 87 (referred to here as 87E, for ethane) that isindependent of whether there is ethane target gas 51 in the region, i.e.along the detection path 59 through which the light beam 67 wastransmitted on the way to the receiver 68. The blank channel data 83Eand the target channel data 87E are supplied from the ethane subsystem74E to the lock-in amplifier 88, the output of which is applied to thedisplay and data processor 89. It may be understood that the splittingof the light beam 67 in this manner by the beam splitter 161 results inthe simultaneous reception by the processor 89 of the data 83M, 83E, 87Mand 87E for simultaneous processing to simultaneously detect theoccurrence of both methane and ethane to assure that the detected gas 51is from a natural gas leak and not from a natural emission source.

It may be understood that the features of the embodiment 50-1 of thesystem 50 may be adapted for separate, non-simultaneous detection ofethane and methane as the target gas 51. For example, when conditions ofthe free atmosphere 52 are stable (relatively invariable as toatmospheric turbulence, etc.), one system 50 (as shown in FIG. 1A or inFIG. 18 as described below) may be provided to detect ethane as a targetgas 51 and another such system 50 may be provided to detect methane as atarget gas 51. The detection paths 59 of each such system 50 may beclose to each other (e.g., within a few inches or a foot).Alternatively, in the system 50 of FIG. 1A one dual channel GCRsubsystem 74 may be provided to detect ethane and an additional dualchannel GCR subsystem 74 may be combined with the ethane system 50 todetect methane. For example, the beam splitter 72 may be movable todirect the two beams 73-1 and 73-2 first into the ethane subsystem 74and then into the methane subsystem 74.

As described above with respect to FIGS. 1A and 1B, the modulatedcollimated light beam 67 has been subjected to the atmosphericconditions along the detection path 59 (e.g., at the successive momentsof time t1, t2, t3, etc.) during which the light beam 67 is transmittedfrom the near end 62 to the far end 63 of the detection path 59. Suchatmospheric conditions further modulate the light beam 67, but thisfurther atmospheric modulation is different from the modulation by themodulator 64. The lock-in amplifier 88 selects from this combination ofmodulated light 58 only the light having the modulation imposed by themodulator 64, such that the data 83 and 87 do not include the effects ofatmospheric turbulence, jitter, beam wander, or changes in the index ofrefraction, for example. Thus, each item of the data 83 and 87 receivedby the processor 89 at any moment of time represents the influence ofthe same current atmospheric conditions of the free atmosphere 52 as thelight 58 is transmitted along the detection path 59, as may have beenmodified only by transmission through the respective channels 76 and 78.As described above, the first detector 82 outputs the blank channel data83 that varies according to whether there is target gas 51 in thedetection path 59 through which the light beam 67 was transmitted on theway to the receiver 68. The second detector 86 outputs target gaschannel data 87 that is independent of whether there is target gas 51 inthe region, i.e. along the detection path 59 through which the lightbeam 67 was transmitted on the way to the receiver 68. The channels 76and 78 thus modify the light beam 67 so that together the data 83 and 87may be processed by the processor 89 to indicate, for example, whetheror not there was target gas 51 along the detection path 59 during thesuccessive moments of time during which the light beam 67 wastransmitted from the near end 62 to the far end 63 along the detectionpath 59.

The lock-in amplifier 88 is configured in two parts, one for the firstchannel 76 and one for the second channel 78. Referring now to FIG. 13,and considering the lock-in amplifier 88-1 for the first channel 76 asan example of the structure and operation for both channels 76 and 78, acrystal oscillator and phase lock-loop circuit 171-1 locks to thefrequency and phase of an input in the form of the blank channel data(or signal) 83 from the first channel 76. The crystal oscillator andlock-in circuit 171-1 generates an output 173 modulated at theappropriate frequency. The output 173 is delivered to a mixer 176. Inaddition, the lock-in amplifier 88-1 accepts a selectable input 172 fromeither the user (from knowledge of the approximate source modulationfrequency) or from the source modulation reference signal 97 (FIG. 1B).A phase-lock-loop circuit 174-1 of the amplifier 88-1 tunes thefrequency of an output 175-1 to that of the selectable input 172. Aphase shifting circuit 174-2 within the amplifier 88-1 adjusts theelectronic phase of the signal 175-1 from the phase lock loop circuit174-1. An output 175-2 from the phase shifting circuit 174-2 is thenprovided to the mixer 176. The mixer 176 within the amplifier 88-1produces the sum and difference frequencies of inputs to the mixer 176(i.e., outputs 173 and 175-2). An output 177 from the mixer 176 ispassed through a low pass filter 178 to remove the sum of thefrequencies contained in the signals 175-2 and 173 and thereby passingonly the difference frequencies. Finally, the signal 177 that passesthrough the low pass filter 178 is amplified by an amplifier 178-1,resulting in a lock-in output signal 179. Thus, the amplified signal 179is proportional to the magnitude of the signal 83 from the blank channel76 that is at the same frequency as the selectable input 172. The signal179 is input to the processor 89. Minor changes to the selectable input172 frequency may be performed under microprocessor control in order tomaximize the output signal 179 by varying the phase lock loop 174-1frequency via control line 180. Phase adjustments may also be performedunder microprocessor control in order to maximize the output signal 179.A signal proportional to the phase adjustment performed by themicroprocessor may be obtained at output 181. Thus, the lock-inamplifier 88-1 outputs a signal 181 representing the phase and thesignal 179 representing the magnitude of the blank channel signal 83relative to the internal reference. Given this measured phase andmagnitude of the blank channel signal 83, the lock-in amplifier 88-1precisely electronically locks to the phase and frequency of the blankchannel signal 83. In a similar manner, the lock-in amplifier 88 for thefirst channel 78 measures the phase and magnitude of the target channeldata 87 and precisely electronically locks to the phase and frequency ofthe target channel data 87. In this manner, (including when the sourcemodulation reference signal 97, FIGS 1B, 17, and 18) cannot be used toprovide the needed phase and frequency) the modulation of the light 58at the source 61 enables the modulated light beam 67, itself, via theblank channel data 83 which was derived from the light beam 67, toprovide the required phase and frequency to the lock-in amplifier 88 ofthe receiver 68. In turn, this eliminates the need to extend cabling, orhard wire, between the modulator 64 at the source 61 and the receiver68. Further, the provision of the lock-in amplifier 88 in the receiver68, in conjunction with the configuration of the dual channel GCRsubsystem 74 for simultaneous channel operation, effectively eliminatesfrom the first and second channel data 83 and 87 the effects on thelight beam 67 of atmospheric turbulence, beam wander and drift, sourceillumination variations, etc. In attempts by other than the presentinvention to detect target gases 51, these atmospheric and systematiceffects often dominate the target gas measurement uncertainty. Thus, theabove-described configurations of the source 61 with the modulator 64,and of the receiver 68 with the lock-in amplifier 88, and of the GCRsubsystem 74, greatly improve the detectable concentration achievable bythe system 50, which may be reduced to the above-described few PPB forethane detection, for example.

As described above, the data 83 and 87 from the respective blank andtarget gas channels 76 and 78 are input to the processor 89. Theprocessor 89 is programmed to perform operations of measurementtechniques to further reduce the impact of competitive gases 53 on theminimum detectable concentration achievable by the system 50. Theprogrammed operations of the measurement techniques also reduce theimpact of broadband scattering, such as produced by aerosols, along thedetection path 59. These programmed operations involve measuring a valueof a “null” signal under similar atmospheric conditions as exist duringthe detection and measurement of the concentration of the trace gas 51.

FIG. 14 shows a flow chart 181 having operations of a method of theinvention for performing such a measurement technique. The methodrelates to Equation (1), which may be described in more detail as thefollowing ratio: $\begin{matrix}{{Output} = {{\frac{{Ratio}_{measured}}{{Ratio}_{null}} - 1} = {\frac{\left( \frac{{Signal}_{gas}}{{Signal}_{blank}} \right)}{\left( \frac{{Signal}_{{gas} - {null}}}{{Signal}_{{blank} - {null}}} \right)} - 1}}} & {{Equation}\quad (9)}\end{matrix}$

where Output represents the output of the processor 89. In the ratio ofEquation (9), Signal_(gas) is the amplitude of the output of thedetector 86 (which is the target gas data 87) measured through the gaschannel 78, and Signal_(blank) is the amplitude of the output of thedetector 82 (which is the blank channel data 83) measured through theblank channel 76. Ratio_(measured) is the ratio formed by dividingSignal_(gas) by Signal_(blank). Ratio_(null) is determined by removingthe target gas cell 84 and the blank cell 81 from the respectivechannels 78 and 76. Under this modified channel configuration,Signal_(gas-null) is the amplitude of the output of the detector 86 (thetarget gas data 87) measured through the gas channel 78, andSignal_(blank-null) is the amplitude of the output of the detector 82(the blank channel data 83) measured through the blank channel 76.Ratio_(null) is then calculated as the ratio of these two values. Thus,Ratio_(null) is based on the differences in the optical and electronicresponses of the structural elements of the respective blank and targetgas channels 76 and 78. Such elements include optics, the detector 86,and typical preamplifier and post-amplifier circuits andanalog-to-digital converters (not shown). Each of the blank channel data83 and target gas channel data 87 is in this manner determined relativeto such null value.

The method of the invention shown in FIG. 14 may start by moving to anoperation 182 in which calibration of the system 50 is performed toobtain the above Output. In detail, in operation 182 the Equation (9)Output, i.e., the (Ratio_(measured)/Ratio_(null))−1, is determined byoperating the system 50 using a series of known,concentration-calibrated gas standards as the target gas 51.Preferrably, such calibration of the system 50 may be performed once thesystem 50 is placed at a particular detection path 59 just prior todetection of an unknown target gas 51. More preferrably, suchcalibration may be performed at the factory as a step in the manufactureof a unit embodying the principles of the system 50.

The method moves to an operation 183 in which a look-up table is createdfrom the above Output. For each known target gas 51, the look-up tablelists the Output of the system 50 and corresponding values of theconcentration of the known target gas 51 in terms of the units of“concentration×pathlength” (PPM-m or PPB-m).

The method moves to operation 184 in which the system 50 is operated atnull status to measure Signal_(gas-null) and Signal_(blank-null) andobtain Ratio_(null). Operation 184 of the method of flow chart 181 isshown in more detail with reference to FIG. 15A. Operation 184 includesa suboperation 192 in which the first and second channels 76 and 78 aremodified to null status and a null-calibration is performed. Suchmodification is performed by removing the target gas cell 84 from thesecond channel 78 and the blank cell 81 from the first channel 76. Also,the system 50 is placed so as to define a particular detection path 59,and the housings 91 and 92 are aligned so that the light beam 67 isaccurately centered as it is received by the receiving telescope 69.Such place may be fixed, e.g., on the ground or on a mobile platform (e.g., on a vehicle described below).

The method moves to suboperation 193 in which light 67 is directedthrough these modified configurations of the channels 76 and 78, toobtain a value of respective Signal_(gas-null) based on the amplitude ofthe output of the detector 86 (the target gas data 87) measured throughthe gas channel 78, and a value of Signal_(blank-null) based on theamplitude of the output of the detector 82 (the blank channel data 83)measured through the blank channel 76. The method moves to an operation194 in which a value of Ratio_(null) is then calculated as the ratio ofthese two values Signal_(gas-null) and Signal_(blank-null). The methodmoves to a suboperation 195 in which the system 50 is returned tooperational status with no such bypasses. The method moves to operation185 (FIG. 14).

In operation 185 the system 50 is operated to detect the target gas 51of unknown concentration. In preparation for this operation, the targetgas cell 84 is configured for detecting such particular target gas 51.Also, the system 50 remains placed so as to define the particulardetection path 59, and the housings 91 and 92 remain aligned so that thelight beam 67 is accurately centered as it is received by the receivingtelescope 69. The system 50 is operated and the Output of the processor89 is obtained.

The method moves to an operation 186 in which this processor Output ofthe system 50 is compared to the data in the look-up table. For example,the Output of the system 50 is used to find the value of theconcentration of the known target gas 51 that corresponds to the Output.This value found in the look-up table is the concentration of The targetgas 51 of unknown concentration expressed in terms of“concentration×pathlength” (PPM-m or PPB-m). The method is DONE.

Details of operation 185 of FIG. 14 are shown in FIG. 15B. Asuboperation 197 operates the system 50 to measure the amplitude, orvalue, of the output of the detector 86 (the target gas data 87) throughthe gas channel 78 to provide a value of Signal_(gas). The method movesto a suboperation 198 in which the amplitude of the output of thedetector 82 (the blank channel data 83) measured through the blankchannel 76 is obtained to provide a value of Signal_(blank). The methodmoves to a suboperation 199 in which a value of Ratio_(measured) isobtained by computing the value of a ratio formed by dividing the valueof Signal_(gas) by the value of Signal_(blank). The method moves to anoperation 201 in which the values obtained in operations 194 and 199 forthe respective Ratio_(null) and Ratio_(measured) are used to compute thevalue of the ratio of Ratio_(measured) to Ratio_(null), i.e., to obtainthe value of the above Output. The method then moves to operation 186,FIG. 14.

One advantage of providing Ratio_(null) in the above Equation (9) is theresulting removal of measurement inaccuracies associated with theelements of the system 50. Specifically, Ration_(null) in Equation (9)removes the effects of alignment variability of the source 61 withrespect to the receiver 68, i.e.,transmitter-to-receiver alignmentvariability. Also removed are variabilities of the responsivities of theblank channel detector 82 and the gas channel detector 86, which are interms of volts of output per optical watt of input. Further removed isvariability of channel electronic gain and variability associated withthe optical characteristics of the blank channel 76 and of the gaschannel 78.

An additional benefit of measuring Ratio_(null) in the field as providedin operations 192 and 193, FIG. 15A (i.e., very close to the time atwhich Ratio_(measured) is determined) is that such measuring ofRatio_(null) eliminates the effect of broad atmospheric absorbers suchas water vapor and aerosols. Such absorbers are broad in the sense thatthey absorb throughout the entire spectral region of interestestablished by the IR filter 71. That is, when Ratio_(null) is measuredin the field, the Output (Ratio_(measured)/Ratio_(null))−1 is largelyindependent of atmospheric water vapor concentration, aerosolconcentration and potentially other interfering trace gas absorption.

Referring now to FIG. 16, an embodiment of the system 50 may beconfigured as follows. The detector 82 for the blank channel 76 is shownin FIG. 16 to illustrate the configuration of both the detectors 82 and86, which are the same except for the light input. The light beam 73-1is input to the detector 82 and the light beam 73-2 is input to thedetector 86 as shown in FIG. 1A. In FIG. 16 the exemplary detector 82 isshown configured with a respective light detector 201 having a responseto the light beam 73-1 that is a function of the light received (i.e.,the intensity of the light), of the detector ambient temperature, and ofthe magnitude of a bias voltage 202 input to the detector 201. A voltagesupply 203 is configured to provide each of the detectors 82 and 86 withthe common bias voltage 202.

As part of this embodiment of the system 50, a temperature stabilizer206 is configured for controlling the ambient temperature of each of thedetectors 82 and 86. This temperature control is applied during alloperations of the system 50 to minimize error due to changes in thesensitivity of the light detectors 201 to the light 58 of the respectivebeams 73, as measured in amperes of output per watt of optical input.The change in such sensitivity of a typical light detector 201 may beapproximately 4% per degree C., and the stabilizer 206 is effective tomaintain the detector temperature within a tolerance not to exceed about0.1 degree C of a desired detector temperature. The stabilizer 206limits the temperature-related drift of the detectors 82 and 86 to about±0.01%, for example.

With the voltage supply 203 and the stabilizer 206 in operation asdescribed, the responses of the two detectors 82 and 86 to therespective light beams 73 are independent of the detector ambienttemperature and the bias voltage 202 input to the respective detector 82and 86, and dependent on the intensity of the respective light beam 73received by the respective detector 82 and 86.

As described above with respect to FIG. 1A, the source 61 of the system50 may be configured to direct the light 58 from the transmissionlocation at the near end 62 of the detection path 59 to the far end 63at the receiver location spaced from the transmission location. Ingeneral, such spacing of the ends 62 and 63 may be from about ten feetto about 5300 feet, which in turn provides a similar selected length ofthe detection path 59. Thus, in the fence line embodiment 50-1 of thesystem 50 shown in FIG. 1A, for example, this detection path length maybe from about 10 feet to about 5300 feet and the separate transmissionhousing 91 and receiver housing 92 are provided at the respective spacedends 62 and 63.

A moderate distance embodiment 50-2 of the system 50 is shown in FIG.17, in which both the transmission housing 91 and receiver housing 92are provided at the same near end 62 of the detection path 59. In theembodiment 50-2 the source 61 may be configured to direct the light beam67 from the transmission location at the near end 62 of the detectionpath 59 along a first length 59-1 of the detection path 59 to a lightbeam reflection location 211 at which a reflective surface 212 may bepresent. Such surface 212 may be a natural surface 212N such as theground or trees, for example, or a non-natural surface 212A such as thatof a wall of a building, for example. The reflective surface 212reflects the light beam 67 out of the first length 59-1 for return tothe receiver 68 at the near end 62 along a second length 59-2 of thedetection path 59. The spacing of the near end 62 and the reflectionlocation 211 may be from about fifteen feet to about 1500 feet, which inturn provides another selected length of the detection path 59. In thismoderate distance embodiment 50-2 of the system 50 the detection pathlength (twice the distance from the near end 62 to the reflectionlocation 211) may be as much as about 3000 feet. A typical configurationof the moderate length embodiment 50-2 is to mount both the transmissionhousing 91 and the receiver housing 92 on a vehicle 216, such as atruck, so that the housings 91 and 92 are provided at the same near end62. The direction of the detection path 59 is typically away from thevehicle 216, such as away from a road on which the vehicle 216 may bedriven, so that any target gas 51 that is away from the vehicle 216along the detection path 59 to or from the surface 212 may be detected.

The embodiment 50-3 of the system 50 is shown in FIG. 18, in which boththe transmission housing 91 and receiver housing 92 are provided at thesame near end 62 of the detection path 59. In the embodiment 50-3 thesource 61 may be configured with a transmitting telescope 221 for longdistance direction of the light beam 67 from the transmission locationat the one end 62 of the detection path 59 along a second long length59-3 of the detection path 59 to a light beam reflection location 222 atwhich a remotely located reflector 223 may be present. Such reflector223 may be designed to reflect the light beam 67 out of the first length59-3 for return to the receiving telescope 69 of the receiver 68 at thenear end 62 along a fourth length 594 of the detection path 59. Thespacing of the near end 62 and the reflection location 222 may be fromabout 15 to about 2600 feet, which in turn provides another selectedlength of the detection path 59. In this embodiment 50-3 of the system50 the detection path length (twice the distance from the near end 62 tothe reflection location 222) may be from about thirty feet to about 5300feet. A typical configuration of the embodiment 50-3 is to mount boththe transmission housing 91 and receiver housing 92 fixed to the groundat a known distance from the reflector 223, for example. With thispossible path length of up to about a mile, the embodiment 50-3 may bereferred to as a long-distance embodiment.

Another long distance embodiment 504 of the system 50 is shown in FIG.19 as combining the vehicle-mounted aspects of embodiment 50-2 and thelong distance detection aspects of the embodiment 50-3. In embodiment504 both the transmission housing 91 and receiver housing 92 areprovided on an airborne vehicle, or aircraft, 230, such as an airplane(or helicopter) at the same near end 62 of the detection path 59. Exceptfor the mounting of the housings 91 and 92 on the airborne vehicle 230rather than on the ground, and the primary use of the ground as areflective surface 231, embodiments 50-3 and 504 are the same. Referringto FIG. 18, the source 61 may be configured with a transmittingtelescope 221 for long distance direction of the light beam 67 from thetransmission location at the near end 62 of the detection path 59. FIG.19 shows the beam 67 directed along a second long length 59-5 of thedetection path 59 to a light beam reflection location 222 which istypically the ground which acts as the remotely located reflector 23 1.Such reflector 231 reflects the light beam 67 out of the first length59-5 for return to the receiver 68 at the near end 62 (FIG. 18) along along return length 59-6 of the detection path 59. The spacing of thenear end 62 and the reflection location 222 may be from about 0 (i.e.when the aircraft 230 is on the ground) to about 2500 feet, which inturn provides another selected length of the detection path 59. In thislong distance airborne embodiment 50-4 of the system 50, the detectionpath length (twice the distance from the near end 62 to the reflectionlocation 222) may be from about 0 feet to about 5000 feet, for example.The direction of the detection path 59 is typically directly downwardaway from the aircraft 230 toward the ground 231 so that any target gas51 that is away from the aircraft 230 along the detection path 59 may bedetected.

Because of the variable conditions along the long lengths of thedetection paths 59 of the embodiments 50-1 through 504, and because ofthe additional reflection conditions which may vary widely as theaircraft 230 flies, each of the systems 50 for these embodiments 50-1through 504 is preferably configured with the above-described featureslisted below to provide the ability to detect a low minimum detectableconcentration of the target gas. The low minimum detectableconcentration may vary according to the IR absorption characteristics ofthe target gas 51. Considering the above-referenced ethane and methanegases as exemplary target gases 51, the system 50 takes advantage of thehigh depth of IR absorption by ethane to provide a lower minimumdetectable concentration of ethane than is provided for methane. Despitethe lower depth of IR absorption by methane (than by ethane), the system50 still provides a minimum detectable concentration of methane (about50 PPB) that is orders of magnitude lower than that of theabove-described prior art instruments.

The listed features include the source 61 provided with the broadbandsource of light 90, and the modulator 64 at the source 61. The lightsource 90 may be the arc lamp or a configuration of a thermal emissionsource (e.g., a plasma glow lamp). The GCR subsystem 74 is provided withthe beam splitter 72 so that the split beams 73-1 and 73-2 are processedsimultaneously to achieve improved measurement sensitivity and accuracy.The IR filter 71 is optimized to have the optimized central wavelengthand the optimized bandpass as described with respect to FIGS. 3A and 3B,which provide substantially increased sensitivity to the particulartarget gas 51 and substantially increased selectivity of such target gas51 to avoid erroneous detection of any competitive gas 53 as the targetgas 51. The lock-in amplifier 88 is configured for generating theamplified signal 179 proportional to the magnitude of the signal 83 fromthe blank channel 76 that is at the same frequency as the selectableinput 172. The detectors 82 and 86 are provided with the stabilizer 206and the voltage supply 203 to improve system sensitivity. Also, asdescribed with respect to the most preferred embodiment of FIGS. 14 and15B, the processing method of obtaining the Output based on Equation (9)using the (Ratio_(measured)/Ratio_(null))−1 is used to removemeasurement inaccuracies associated with the elements of the system 50,variabilities of the responsivities of the blank channel detector 82 andthe gas channel detector 86, as well as the variability of channelelectronic gain and variability associated with the opticalcharacteristics of the blank channel 76 and of the gas channel 78.Finally, Ratio_(null) is measured in the field (i.e., very close to thetime at which Ratio_(measured) is determined) to eliminate the effect ofthe broad atmospheric absorbers.

It may be understood that other embodiments of the system 50 may also beconfigured for use in detecting target gases 51 along a short detectionpath 59, e.g., of from about 10 feet to about 100 feet in length. Also,the housings 91 and 92 may be stationary and in the fence lineconfiguration shown in FIG. 1A. The embodiment 50-5 is referred to as ashort detection path stationary embodiment of the system 50. In view ofthe reduced length of the detection path 59 and the lack of adversereflection conditions (that may vary widely, as in the mobileembodiments 50-2 or 504), one may not need to include in the shortdetection path stationary embodiment 50-5 of the system 50 one or moreof the last mentioned features.

In more detail, for example, in addition to the factors of atmosphericconditions along the detection path 59, the length of the detection path59 is a factor related to the detection, determination, and measurementof the target gas 51. The longer the detection path 59, the more theatmospheric conditions along the detection path 59 may absorb, scatter,or reflect the light beam 67 that is directed along the detection path59, and the more such conditions may temporally vary the light of thebeam 67. Thus, the relatively short detection paths 59 described withrespect to embodiment 50-5 may be subject to different (less severe)atmospheric conditions than the longer detection paths 59 described withrespect to FIGS. 18 and 19, for example. The atmospheric conditions towhich the respective embodiments 50-3 and 504 of the system 50 of FIGS.18 and 19 may be exposed render it more necessary to have the featuresof the system 50 to obtain a minimum detectable concentration of the fewPPB (for ethane, for example). Also, surfaces 212N (FIG. 19) ofnaturally occurring objects, such as the surface of the ground, or thesurfaces of buildings or other things made by people (e.g., thereflector 222, FIG. 18), may have differing light reflectingcharacteristics. Thus, the detection path situation described withrespect to FIG. 1A (having no reflective surface 212 along the detectionpath 59) may present different (less severe) detection, determination,and measurement circumstances due to the respective reflective surfaces223 and 231 shown in FIGS. 18 and 19. Similarly, the long detectionpaths 59 shown in FIGS. 18 and 19 may present a combination ofcircumstances based on both the reflective and the atmosphericconditions, both of which present different (more severe) detection,determination, and measurement circumstances that render it morenecessary to have the features of the system 50 to obtain such a minimumdetectable concentration of a few PPB.

In view of the foregoing description, it may be understood that thepresent invention fills these above-described needs by providing thedescribed methods of and system 50 for distinguishing between the targetgas 51 and other gases 53 that are normally in the free atmosphere 52 atthe same time and in the same place as the target gas 51. The presentinvention fills such needs for trace gas detection in the natural gasindustry by an ability to distinguish between natural gas as a trace gas51 and other combustible gases 53. Also provided is a more optimalnatural gas detector that simultaneously detects both methane and ethaneto assure that the detected methane is from a natural gas leak, so as toavoid false natural gas alarms based on the detection, for example, ofleaking propane tanks, etc. Further, the method and system 50substantially increase the detection distance by providing the describeddetection along the long detection paths 59. Thus, by the presentinvention the distance from the system 50 to a location of the targetgas 51 that is to be detected may be up to about one mile. As a result,mobile platforms, such as the trucks 216 and the aircraft 230, may beused to carry the system 50 during high-speed remote monitoring alonglong detection paths 59. In addition, the system 50 provides highsensitivity to the target gas 51 independently of atmospheric turbulenceand variability, and the above-described undesired influences areremoved from the Output of the processor 89 so as to isolate the Outputrepresenting the trace gas 51. It may also be understood that in thesubsystem 74 the channels 76 and 78 are configured so that thesimultaneously measured data 83 and 87 vary from each other basedsubstantially only on the presence of the target gas 51 in the detectionpath 59 at the time the broadband modulated light 67 is directed intothe free atmosphere 52 along the detection path 52.

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced within the scope of theappended claims. Accordingly, the present embodiments are to beconsidered as illustrative and not restrictive, and the invention is notto be limited to the details given herein, but may be modified withinthe scope and equivalents of the appended claims.

What is claimed is:
 1. A method of optimizing a response of a gascorrelation radiometer to a trace amount of a target gas present in thefree atmosphere along a detection path to the gas correlationradiometer, wherein the detection path may also contain at least onecompetitive other gas the presence of which in the free atmosphere mayinterfere with detection of the trace amount of the target gas, the gascorrelation radiometer using an infrared filter for the response, themethod comprising the operations of: determining an absorption spectrumof the target gas modeled according to field parameters; determining anabsorption spectrum of the at least one competitive gas modeledaccording to the field parameters; determining similarity and contrastbetween the absorption spectra of the atmosphere and of the target gas;determining differences between respective values of contrast andsimilarity corresponding to a plurality of band passes and centerwavelengths of possible infra red filters; for each of the plurality ofband passes, plotting the differences as a function of centerwavelength; and optimizing the infrared filter to be used in the gascorrelation radiometer by selecting a combination of infrared filtercenter wavelength and band pass that results in the largest value ofcontrast minus similarity for the trace gas present in the freeatmosphere along the detection path containing the at least onecompetitive other gas.
 2. A method as recited in claim 1, furthercomprising the operation of: mounting the optimized infrared filter inthe gas correlation radiometer.
 3. A method as recited in claim 1,wherein the at least one competitive gas is water vapor and wherein thedetermining of the second absorption spectrum determines the secondabsorption spectrum corresponding to the water vapor.
 4. A method asrecited in claim 1, wherein the at least one competitive gas is watervapor and another gas, and wherein the determining of the secondabsorption spectrum determines the second absorption spectrumcorresponding to both the water vapor and the another gas.
 5. A methodas recited in claim 1, wherein the method optimizes respective responsesof each of two gas correlation radiometers to trace amounts of therespective target gases ethane and methane present in the freeatmosphere along the detection path to the two gas correlationradiometers, wherein for the gas correlation radiometer for ethanedetection the at least one competitive gas is a gas other than theethane; wherein for the gas correlation radiometer for methane detectionthe at least one competitive gas is a gas other than the respectivemethane; the method further comprising: performing the method of claim 1once with respect to ethane as the target gas and once with respect tomethane as the target gas so that there are provided two optimizedinfrared filters each having the selected center wavelength andbandwidth for use with the respective ethane and methane gas correlationradiometers to filter light transmitted through the free atmosphere tothe respective ethane and methane gas correlation radiometers.
 6. Amethod of optimizing a response of a gas correlation radiometer to atrace amount of a target gas present in the free atmosphere along adetection path to the gas correlation radiometer, wherein the detectionpath may also contain at least one competitive other gas the presence ofwhich in the free atmosphere may interfere with detection of the traceamount of the target gas, the method comprising the operations of:identifying a spectral region of a first absorption spectrum of thetarget gas, the spectrum corresponding to selected parameters of targetgas concentration, target gas temperature, target gas pressure, and pathlength through the target gas, the spectral region having a plurality ofhigh absorption characteristics and low absorption characteristics forthe spectral region, providing a second absorption spectrum of the atleast one other competitive gas, the second absorption spectrumcorresponding to the selected parameters and including non-absorbingregions corresponding to the low-absorption characteristics of the firstabsorption spectrum; determining a set of similarity data as a functionof overlap regions within the spectral region, the overlap regions beingfor each of the at least one other competitive gas and the target gasand being those regions within the spectral region in which therespective absorption spectra of both the target gas and the at leastone other competitive gas have absorption characteristics, the set ofsimilarity data including a plurality of data items within each of aplurality of bandwidths, one of the data items corresponding to a centerwavelength within each bandwidth; determining a set of contrast data asa function of non-overlap regions within the spectral region, thenon-overlap regions being for each of the at least one other competitivegas and the target gas and being those regions within the spectralregion in which the first absorption spectrum has high absorptioncharacteristics but the second absorption spectrum has low absorptioncharacteristics, the set of contrast data including a plurality of dataitems within each of a plurality of bandwidths, one of the data itemscorresponding to a center wavelength within each bandwidth; preparing agraph corresponding to each of the bandwidths, each graph being a plotof the data points, each data point having an ordinate axis value basedon a contrast data item minus a similarity data item, each data pointhaving an abscissa axis value based on one of the center wavelengths;from one of the prepared graphs, selecting as a center wavelength of aninfrared filter for use with the gas correlation radiometer the centerwavelength corresponding to the highest value of the contrast data itemminus the similarity data item of all of the graphs; selecting as thebandwidth of the infrared filter the bandwidth corresponding to thegraph having the highest value of the contrast data item minus thesimilarity data item of all of the graphs; and providing the infraredfilter having the selected center wavelength and bandwidth for use withthe gas correlation radiometer to filter light transmitted through thefree atmosphere to the gas correlation radiometer.
 7. A method asrecited in claim 6, wherein the at least one competitive gas is watervapor and wherein the providing of the second absorption spectrumprovides the second absorption spectrum corresponding to the watervapor.
 8. A method as recited in claim 6, wherein the at least onecompetitive gas is water vapor and another gas, and wherein theproviding of the second absorption spectrum provides the secondabsorption spectrum corresponding to both the water vapor and theanother gas.
 9. A method as recited in claim 6, wherein the methodoptimizes respective responses of each of two gas correlationradiometers to trace amounts of the respective target gases ethane andmethane present in the free atmosphere along the detection path to thetwo gas correlation radiometers, wherein for the gas correlationradiometer for ethane detection the at least one competitive gas is agas other than the ethane; wherein for the gas correlation radiometerfor methane detection the at least one competitive gas is a gas otherthan the respective methane; the method further comprising: performingthe method of claim 6 once with respect to ethane as the target gas andonce with respect to methane as the target gas so that there areprovided two optimized infrared filters each having the selected centerwavelength and bandwidth for use with the respective ethane and methanegas correlation radiometers to filter light transmitted through the freeatmosphere to the respective ethane and methane gas correlationradiometers.
 10. A method as recited in claim 6, wherein the operationof determining the set of similarity data as a function of overlapregions within the spectral region is performed using Equation (1)below, wherein: λ₂ and λ₁ define the bandwidth, T_(atm) is thewavelength-dependent optical transmission through the atmosphere,T_(cell) is the wavelength-dependent optical transmission through atarget gas channel of the radiometer, T_(filter) is thewavelength-dependent optical transmission through the filter, and ∂λ isan increment of wavelength within the bandwidth of the infrared filter:$\begin{matrix}{{similarity} \equiv {\int_{\lambda_{1}}^{\lambda_{2}}{{T_{filter}\left( {1 - T_{atmosphere}} \right)}\left( {1 - T_{cell}} \right){{\partial\lambda}.}}}} & {{Equation}\quad (1)}\end{matrix}$


11. A method as recited in claim 6, wherein the operation of determiningthe set of contrast data as a function of overlap regions within thespectral region is performed using Equation (1) below, wherein: λ₂ andλ₁ define the bandwidth of the infrared filter, T_(atm) is thewavelength-dependent optical transmission through the atmosphere,T_(cell) is the wavelength-dependent optical transmission through atarget gas channel of the radiometer, T_(filter) is thewavelength-dependent optical transmission through the filter, and ∂λ isan increment of wavelength within the bandwidth of the infrared filter:$\begin{matrix}{{contrast} \equiv {\int_{\lambda_{1}}^{\lambda_{2}}{{T_{filter}\left( T_{atmosphere} \right)}\left( {1 - T_{cell}} \right){{\partial\lambda}.}}}} & {{Equation}\quad (1)}\end{matrix}$


12. A method as recited in claim 6, wherein: the operation ofdetermining the set of similarity data as a function of overlap regionswithin the spectral region is performed using Equation (1) below; andwherein the operation of determining the set of contrast data as afunction of overlap regions within the spectral region is performedusing Equation (2) below; and wherein λ₂ and λ₁ define the bandwidth ofthe infrared filter, T_(atm) is the wavelength-dependent opticaltransmission through the atmosphere, T_(cell) is thewavelength-dependent optical transmission through a target gas channelof the radiometer, T_(filter) is the wavelength-dependent opticaltransmission through the filter, and ∂λ is an increment of wavelengthwithin the bandwidth of the infrared filter: $\begin{matrix}{{{similarity} \equiv {\int_{\lambda_{1}}^{\lambda_{2}}{{T_{filter}\left( {1 - T_{atmosphere}} \right)}\left( {1 - T_{cell}} \right){\partial\lambda}}}},} & {{Equation}\quad (1)}\end{matrix}$

$\begin{matrix}{{contrast} \equiv {\int_{\lambda_{1}}^{\lambda_{2}}{{T_{filter}\left( T_{atmosphere} \right)}\left( {1 - T_{cell}} \right){{\partial\lambda}.}}}} & {{Equation}\quad (2)}\end{matrix}$